What is a crucial requirement for keeping tank cars that are not being loaded?

Keeping tank cars that are not being loaded coupled together is important because it enhances safety and stability during storage. Coupling the cars ensures that they are less likely to roll away if they are parked on a slight incline, which can pose a significant safety risk. Coupled cars are also easier to manage and move as a unit if they need to be positioned or relocated for operational reasons.

While other options may encompass certain safety or operational aspects, they do not prioritize the structural integrity and stability of the tank cars in the same way coupling does. For instance, uncoupling the cars may increase the risk of unintended movement, and moving them to a staging area without coupling might add unnecessary complexity to their management. Additionally, emptying the contents immediately is not typically a requirement unless specific operational protocols dictate so, making coupling the most logical and crucial requirement for maintaining safety and organization for tank cars that are not being loaded.